Gene transfer experiments can be used to identify and characterize oncogenes. We are developing a rat donor/mouse recipient gene transfer system with broad utility for studying oncogenes from cells transformed by chemical carcinogens. The first requirement is a probe (cloned DNA fragment) that will hybridize with any rat DNA segment of reasonable length and will not hybridize to mouse DNA. We have developed such a probe and are presently confirming its utility in model experiments with donor DNA from virally transformed rat cells. This probe is a composite of different repeated sequences. All of them are interspersed in the rat genome and each of them hybridizes to a substantial fraction of recombinant phages in a rat library. Some of them are transcribed in a tissue-specific manner. Thus, when oncogenes are detected in transformed cells, the composite probe can be disassembled into components to determine which family of repeated sequences has a member contiguous with each oncogene. If transcription of adjacent genomic segments is required for expression of oncogenes, this will be apparent. Thus, we have developed a probe that can detect oncogenes and provide information about them in a single step.